1. Field of the Invention
This invention relates to electrical devices comprising conductive polymers, and in particular to the provision in such devices of highly conductive layers to which electrical leads can readily be attached.
2. Summary of the Prior Art
Conductive polymer compositions [including such compositions which exhibit positive temperature coefficient (PTC) or negative temperature coefficient (NTC) behavior] and electrical devices comprising them, are known. Reference may be made for example of U.S. Pat. Nos. 2,978,665 (Vernet et al), 3,243,753 (Kohler), 3,311,862 (Rees), 3,351,882 (Kohler et al), 4,017,715 (Whitney et al), 4,085,286 (Horsma et al), 4,095,044 (Horsma et al), 4,177,376 (Horsma et al) and 4,177,446 (Diaz) and to copending and commonly assigned applications Ser. Nos. 818,711 (Horsma et al), 963,372 (Horsma), 965,343, now U.S. Pat. No. 4,237,441, (Van Konynenburg et al), 965,344, now U.S. Pat. No. 4,238,812, (Middleman et al), 969,928 (Van Konynenburg et al), 6,773 (Simon), 38,218 (Middleman et al), 41,071 (Walker) and the continuation-in-part thereof filed July 10, 1980, 75,413 (Van Konynenburg), 84,352 (Horsma et al), 85,679 (Toy et al), 88,344 (Lutz), 98,711 (Middleman et al), 98,712 (Middleman et al), 102,576 (Brigham), the application filed by Brigham on Dec. 7, 1979, now Ser. No. 102,621, 134,354 (Lutz), 141,984 (Gotcher et al), 141,987 (Middleman et al), 141,988 (Fouts et al), 141,989 (Evans), 141,990 (Walty), 141,991 (Fouts et al), 142,053 (Middleman et al), and 142,054 (Middleman et al). The disclosure of each of these patents and patent applications is incorporated herein by reference. The term "conductive polymer" composition is used herein to denote a composition which has a resistivity of less than 10.sup.6 ohm.cm at a temperature between 0.degree. C. and 200.degree. C., preferably at 25.degree. C.
In many such devices, current is passed through the conductive polymer by means of laminar electrodes, and the electrical leads to the remainder of the circuit are attached to the electrodes. The electrodes are generally composed of a material having a resistivity of at most 5.times.10.sup.-2 ohm.cm, preferably a metal, and may be (but are not necessarily) of a thickness such that all points on any particular electrode are at the same potential. When the devices are subject to temperature cycling, differences between the thermal coefficients of expansion of the electrode materials and the conductive polymers tend to result in separation of the electrode from the conductive polymer element. This is of course highly undesirable. The problem is particularly severe when the conductive polymer element comprises a PTC or NTC conductive polymer element, since the PTC or NTC effect depends upon a change in the volume of the PTC or NTC element. It is, therefore, preferred to use an electrode which can expand and contract with the conductive polymer, especially an electrode having a plurality of apertures therein e.g. a metal mesh or grid, the apertures of the electrode preferably being of a size such that the conductive polymer can penetrate into the apertures and anchor the electrode and the conductive polymer to each other. Unfortunately, however, there are serious problems in securing electrical leads to these preferred electrodes. Thus it is unsatisfactory to solder or weld the lead to a portion of the electrode which is contacted by the conductive polymer, inter alia because the soldering or welding process degrades the polymer. This can be avoided by soldering the lead to a portion of the electrode which extends beyond the edge of the conductive polymer; but this leads to a device of greater size and to waste of electrode material, and severely restricts the range of manufacturing techniques.
One method of attaching a lead to a portion of a laminar apertured electrode which is in contact with a conductive polymer is described in Application Ser. No. 141,990 (Walty). The leads are attached, e.g. by soldering, to an apertured conductor, which is then bonded by means of a conductive adhesive to an area of the electrode which is contacted by the conductive polymer; a layer of polymeric material is then placed over the conductor and penetrates through the openings thereof to contact and bond to the conductive polymer. Although this is a very useful technique, it is somewhat inconvenient and expensive and does not give a satisfactory result for all purposes.
As described in detail below, the present invention makes use of flame-sprayed layers of metal or other highly conductive material as a means for making electrical contact with conductive polymer elements. The term "flame-spraying" is used in this specification to denote any process in which a material is brought to its melting point and sprayed onto a surface to produce a coating. Thus the term includes the processes which are known in the art as the metallizing, "Thermospray" and plasma flame processes, as described for example in 1967 Bulletin 136C and other publications of Metco Inc., Westbury, N.Y. In the metallizing process, a metal wire is melted in an oxygen-fuel-gas flame and atomized by a compressed air blast which carries the metal particles to the surface. The "Thermospray" process is similar except that the material is supplied as a powder and may be a metal or non-metal. The plasma flame process is similar to the "Thermospray" process, but makes use of a plasma of ionized gas to melt the powdered material and convey it to the surface. Flame-sprayed coatings have been used for a wide variety of purposes, including the provision of solderable electrical connections to carbon resistors and brushes and to ceramic materials, including PTC ceramics used in thermistors, such as barium titanates ]see the 1967 Metco Bulletin 136C, U.S. Pat. No. 3,023,390 (Moratis et al) and U.S. Pat. No. 3,676,211 (Kourtesis et al)]. Moratis et al flame-spray an alloy containing 40-55% silver, 20-30% cadmium, 10-20% zinc and 10-20% copper, and according to Kourtesis et al, that procedure "while resulting in an acceptable ohmic contact, is prohibitively PG,6 expensive because of the high silver content of the alloy and results in a contact having an insufficient bond between the alloy and the ceramic". Kourtesis et al. teach that the ceramic material must be preheated to a temperature of 450.degree.-525.degree. F. before being flame-sprayed, in order to minimize thermal shock problems. Flame-sprayed metal layers have also been used to provide electromagnetic shielding on the surfaces of cabinets made of insulating polymeric materials.